The contra-rotating rotors type helicopters sometimes erroneously are considered to be free from anti-torque action of the rotors on the helicopter fuselage while in the reality, due to the aerodynamic interferences between the rotors in various regimes of flights, the balance between the driving torques of the rotors is being upset, calling for the need of automatic yaw stabilization.
Several concepts for the yaw control of such helicopters have been devised, such as those covered by patents issued to Maillard, Huvers, Garfinkle, Korsak etc. (see References). All of these concepts can provide the balance of driving torques of contra-rotating rotors by means of some kind of a differential gearing, however their problems is that for the helicopter yaw control, to upset the torque equilibrium, these concepts require other devices, such as fluid motor/pumps or electric motor/generators or brakes etc. which add to the complexity of the yaw control systems.